Find the vertex of the graph of the equation y = -2x^2 + 8x - 15 - 2x^2 + 12x - 7.
y = –2x2 + 8x – 15 – 2x2 + 12x – 7
y = –4x2 + 20x – 22
take 1st derivitive y' = –8x + 20
set 1st deriv equal zero –8x + 20 = 0
–8x = –20
x = 2.5 <<— this is the x-coordinate of the vertex
The first derivitive is the slope of the curve at any point. When the slope is zero, that
means there's no slope, i.e., the curve is turning around and going the other direction.
plug 2.5 into equation y = (–4)(2.52) + (20)(2.5) – 22
y = –25 + 50 – 22
y = 3 <<— this is the y-coordinate of the vertex
so the vertex is located at (2.5, 3)
.